(1) Field of the Invention
The present invention relates to camera systems, and more particularly, to a method and apparatus to control color correction and provide neutral density control of the incoming light through the optical path.
(2) Art Background
Cameras are well known and widely used. There are many types of cameras. While the principle of operation varies, each type of camera records an image formed by rays from the electro-magnetic spectrum which is typically the range of the electro-magnetic spectrum that can be perceived by the human eye. Generally, a camera is a closed box which contains a lens, or combination of lenses, and a sensor which receives and record rays of light passing through the lens, thus fixing an image of an object in focus. One kind of camera records an image chemically on a sensitized plate or film. Another kind of camera records an image electronically by generating a video signal from a pickup tube (e.g. iconoscope, Orthicon) or solid-state counterpart (e.g. charge-coupled image sensor (CCD)). To avoid unnecessarily confusing the presentation of the invention, a "film" camera will refer to the former type of camera and a "video" camera will refer to the latter.
Cameras can also be classified by whether they capture a single image at a time or depict motion by capturing a sequential series of images. Still cameras capture a single image on film and video-store cameras capture a single video image. Motion picture cameras store a series of images on film and television cameras use video to capture a series of images.
The light-gathering ability of a camera is determined by the diameter of the lens. The larger the lens diameter, the greater the amount of light falling on the target that enters the camera. An iris in the lens varies the opening for light to enter the lens. The light input is controlled by controlling the iris. One important factor affected by the lens opening is the depth of field, the distance between the object in focus closest to the camera and that object at a distance farthest from the camera while remaining in focus.
At wide lens openings, the depth of field is poorest. If the depth of field is to be increased, the lens must be "stopped down" (i.e. the iris aperture diameter reduced) and the amount of light on the subject increased to compensate for the smaller aperture. If the lighting level remains constant, still picture photographers can increase the exposure time, trading exposure speed for depth of field, provided there is little or no movement in the picture. However, the exposure time is fixed at the frame rate for both motion picture and television cameras.
In any type of camera, the amount of light reaching the sensor when the aperture size changes can be controlled by neutral density filters. Neutral density filters decrease light transmission uniformly across the visual spectrum. Neutral density adjustment is commonly accomplished by installing gray filters of fixed attenuation values in the optical path of the camera. The neutral density filters are installed singularly, or a group may be used in a rotary cassette. Neutral density filters are also commonly used in film cameras to down-rate the film speed when a camera loaded with fast film is suddenly used to photograph a brightly lit scene.
Human vision corrects for differences in illumination quite automatically. A shirt, for example, is accepted as white both indoors under incandescent lighting and outdoors under natural light. Film and video cameras are not self-adjusting in that sense and color correction must be provided to compensate for the change in color caused by a change in illumination.
A color television camera can be thought of as three cameras in one housing, one for each primary color: red, green and blue. A typical studio camera contains three pickup tubes, one for each primary color. An optical separator, behind the main lens (the taking lens), breaks incoming light into its red (R), green (G) and blue (B) values. Separate preamplifiers and processors handle these R, G and B signals.
Optical separation of the light into its three components may be achieved in different ways. In a simple optical separator with color filters, incoming light from the taking lens is split into three light paths, one per pickup tube, by partially silvered mirrors. The mirrors pass part of the light and reflect the remainder. In front of each pickup tube is an optical filter. These filters are selected to pass a narrow band of wave lengths centered on the red, green and blue primary signals. The filters block unwanted light. The blue filter, for example, passes blue frequencies but blocks red and green frequencies. However, there is excessive light loss in a simple optical separator with color filters.
Dichroic mirrors solve the light-loss problem in an optical separator because they pass certain wave length bands and reflect others. In a dichroic mirror optical separator, the first mirror reflects blue light, but passes the remainder. The blue light is totally reflected from a front-silvered mirror into a relay lens, which forms an image of the blue components of the picture on the target plate of the blue pickup tube.
The light that passes through the first dichroic mirror then impinges on a second mirror. Here the red component is reflected, and the remainder passes through. What is left is white minus red and blue, which is essentially green. Very little of the red-green-blue component is lost. The light that reaches the red pickup tube is a very large fraction of the total red light leaving the taking lens.
A variation of the dichroic mirror system makes use of prisms instead of mirrors. In this system compound series of critically ground and coupled glass prisms separate incoming light into red, green and blue components. There is no glass-to-air interface within the prism system, resulting in less light loss as a result of scattering. Thus prism systems offer greater light sensitivity. For this reason, the use of prisms for optical separation is generally preferred over the use of mirrors.
A color TV camera is considered balanced for a particular reference white when a neutral white card is illuminated with the lighting to be used for shooting and the red, green and blue channels provide equal output levels.
If the light source is changed, such as in going from the studio to an outdoor setting, the camera must be rebalanced. The gains of the three channels must be readjusted to provide the same output on the white card with a new source of light (in this case, the sun). The usual practice is to hold the gain of the green channel fixed and alter the red and blue gain to match the red and blue amplitude to the green amplitude. However, increasing gain, especially blue gain, produces a noisy picture which may be unacceptable.
Color correction can also be accomplished in any color camera by installing color band pass filters of fixed frequency in the optical path to reduce the amount of light of a particular frequency band. The color band pass filters may be installed singularly, or a group may be used in a rotary cassette.
Cameras that use optical filters to control neutral density light transmission and color correction are limited in the corrections which can be made. The corrections may only be made in discrete increments based upon the number of filters available. Therefore, while film motion picture cameras can maintain continuous control of depth of field, a color television camera is limited to a discrete number of field depths for a given light level. Furthermore, only a discrete number of light levels may be used with a color television camera due to the limited number of color correction filters available at any given time. Also, the process for determining the proper filter or series of filters to use can be a cumbersome and time consuming task. To ease the process, some cameras are provided with a rotary cassette containing a number of filters. To change a filter, the cassette is simply rotated to position a new filter. However, the number of filters contained in each cassette is limited, limiting the range of filtering that can be performed. Moreover, even when filters are contained in a rotary cassette, there is a delay as the filters are changed.